First Demonstration Structures Using A New, Carbon-Negative Building Material

David Stone, PhD
November 1, 2010

Portland cement has been called the most important material in the world because it is used so extensively in construction. It literally is the foundation for building the modern world. Yet, it has a very serious problem for the by-product of making Portland cement is carbon dioxide and lots of it. For every ton of cement approximately one ton of CO2 is also produced and there is no way to avoid this since it is an inextricable part of the basic reaction. Because of the major role this greenhouse gas plays in global warming the search is on for alternative cements that are carbon neutral or, better, carbon negative and trap CO2.

One such potential alternative is an iron-based composite I invented while doing research at the University of Arizona. It is truly carbon negative and, in fact, will only harden when exposed to high concentrations of CO2, which diffuses into the wet paste, reacts with the iron, and then becomes trapped as iron carbonate. So the gas actually ends up incorporated into the solid mineral matrix that cements the aggregate together. In a similar way, water is incorporated into Portland cement, which is why it is called a hydraulic cement. This new material might be called a carbonic cement, since CO2 rather than water is the reactive agent that causes cementation.

This process, however, is not simple carbonation but a type of corrosion, which commonly causes the formation of iron oxides (rust), the result of iron combining with oxygen in the presence of water. But corrosion can be caused by other reactants besides oxygen. When CO2 dissolves into water it forms carbonic acid, also known as seltzer water and the basis for many ‘soft’ drinks, though it is surprisingly hard on iron. The Latin roots of the word “corrode” mean to be eaten away and that is what happens to the tiny iron particles in the paste. Iron itself dissolves from their surfaces into the acidic solution. But then the free iron quickly combines with carbonate molecules and precipitates back out of solution as solid iron carbonate. Its crystalline structure identifies it as being a form of the mineral siderite (from the Greek sideros, meaning ‘iron stone’). This is a classic example of the core-shell process, where a shrinking core feeds the growing shell around it. In this case, the hard iron carbonate shells around the iron particle cores gradually grow together and turn the mushy paste into a rock-like mass.

Here is an image of a piece of the cured material, which has been broken to show the inner texture. The aggregate in this specimen is masonry sand. The brown color is characteristic of the oxidized form of the mineral siderite. A cylindrical sample from the same batch was crushed in a testing machine, which showed that it had greater compressive strength than mortar made with Portland cement (7,200 psi). The cured paste without sand failed at an even higher pressure, over 10,000 psi, which is the industry threshold for a “super-strong” cement.

I eventually discovered that various industrial waste steel dusts are available such as spent abrasives that work well as the main ingredient. Some of them have no other use and are typically discarded at landfills so this process is a unique way for them to be recycled. Crushed glass can be used as an aggregate since there is no destabilizing ‘alkali silica reaction’ as with Portland cement. Also, other ingredients were found that function as promoters for the cementation reaction, catalysts for iron carbonate formation, and stabilizers for the product. Finally, it was confirmed that the by-product of the primary reaction is hydrogen gas, which might be harvested from large-scale operations. So overall this is a process that recycles wastes, sequesters carbon dioxide, and generates a clean fuel while producing a building material.

The first demonstration building has been completed. The iron composite was used to form the walls and roof of a small but massive greenhouse. The 8” thick walls were laid up in batches by hand without forms by filling in between panels of steel rod. Tubes of wire mesh were embedded in the walls for delivery of CO2 so the material could be carbonated from the inside out. To facilitate a faster and more thorough cure, the mix was made very porous by adding more chunky glass aggregate than could be filled in with paste, leaving interconnected space that the gas could flow through. The images below show the wall structure under construction, left, and completed. The green hoses connect the internal horizontal tubes into one continuous line. Initially, pure, compressed CO2 in tanks was used but it was found that the exhaust from a small engine also worked well.

The roof of the greenhouse was also made of the iron cement though was done in a very different way than the walls. It is a so-called thin shell structure, about 1.5 inches thick, though highly reinforced with steel rod, wire, and fibers. This type of simple composite, called ferrocement, has been used with Portland cement-based mortar to make hulls for boats including ocean-going ships. Here even the cement paste itself is iron-based so it really is ferrocement. Rather than being carbonated from the inside out, the much denser roof shell was covered above and below with plastic sheets and the enclosed space was filled with CO2 under continuous flow. After 10 days the material appeared completely cured, was very stiff, and did not deflect when loaded with 200lbs/ft2. It will be a green roof and therefore is expected to support the extra weight of several inches of wet soil as well as several feet of wet snow since it will not shed it.

The images below show the roof under construction, left, with the reinforcement just beginning to be filled in with iron cement mortar and, on the right, with the troweling almost complete. Yes, that is David on the roof!

The basic roof shell will later be covered with other layers including foam insulation, a water-proof membrane, a drainage medium, a water-permeable fabric that holds back soil and roots, a growing medium, and finally plants, which will be shallow-rooted, low-growing succulents.

This material technology and construction system appear to have the potential to be successfully developed for commercial applications. At this point, the highest priority is the exploration of other building applications with a focus on the full development of its inherent potential. For example, it may be possible to continuously recharge the material so that the walls function both structurally and as part of an energy system that converts solar DC into hydrogen gas as fuel.